Due to the high prices of oils and regulation of exhaust gases throughout the world, environment-friendly policies and improvement of fuel efficiency have become core goals in the development of vehicles. Accordingly, the vehicle manufacturers are making a lot of efforts to develop technologies for reducing both use of fuels and generation of exhaust gases for the environment-friendly policies and high fuel efficiency.
Under the background, many interests and efforts are being made in the development of technologies such as hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs), which combine the power of an engine and a motor for high fuel efficiency.
The hybrid electric vehicles satisfy purchase needs of many customers due to the high fuel efficiency and environment-friendly images.
However, in the hybrid electric vehicle, engine efficiency decreases as the engine is unconditionally driven for diagnosis and warming-up when the vehicle is initially started after being parked, and fuel efficiency eventually deteriorates as discharging-oriented control for adjusting a state-of-charge (SOC) of the vehicle to a reference SOC when the vehicle is restarted after being parked.